Magnesium plays an essential role in cellular metabolism, functioning as a cofactor for kinase, phosphatases, and synthetases in all cellular compartments. Additionally, hypomagnesemia has been demonstrated to be associated with cardiovascular disease and diabetes. However, methodology for determining cysotolic levels of free magnesium ions has been limited. A number of recent studies have utilized in vivo phosphorus-31 nuclear magnetic resonance (NMR), which provides information based on the equilibrium between free and magnesium complexed ATP. However, there are several important limitations to this technique, most significantly the relatively low dissociation constant of ATP for magnesium such that the cellular ATP pool is generally almost fully complexed with magnesium. Fluorinated, intracellular chelators have recently been shown to be useful for the determination of cytosolic calcium levels using fluorine-19 NMR. During the past year we have designed, synthesized, and tested several fluorinated chelators which have dissociation constants suitable for the determination of basal cytosolic magnesium levels. The indicators developed are fluorinated analogs of o-aminophenyol-N,N,O-triaetic acid (APTRA). Magnesium dissociation constants determined for the 4-fluoro, 5-fluoro-, and 4-methyl, 5-fluoro derivatives of APTRA are 10, 3, and 1.5 mM, respectively. Preliminary measurements have yielded values of 0.3 mM free magnesium for erythrocytes, and 3.0 mM for perfused rat heart. Work is currently in progress to evaluate these indicators for measurements in different systems, and to develop analogous fluorescent magnesium indicators.